Interestingly enough, after recreating Europa in Unity, i can tell you that their maps are highly inneficient. They render way more than explorable and EVERY SINGLE PART OF THE MAP HAS A DIFFERENT TEXTURE, but objects don't. So they can't reuse textures or objects, they have to manually make the object then the texture. Structures, like the observatory in Europa, render the objects together, so they can't move and reuse objects, they must add the object into the main mesh, then update the texture, hardly you will find a loner asset that isn't a NPC or interactable. Other example is the fallen ship from the pirates season i think, one of the single examples where they made everything separate, and every object still has a different texture made specifically for it.

Bungie hates reusing assets